The invention relates to arrangement in connection with an anaesthesia/ventilation system for a patient comprising means for flowing inspiratory gas to the patient and means for flowing expiratory gas from the patient to a gas separation means and further through the gas separation means back to the inspiratory flow.
Referring to the basic principles of anaesthesia/ventilation technique it is important to understand that only a part of the anaesthetic agent inhaled by a patient is absorbed in the alveoli. The excess goes to the atmosphere. This is both expensive and bad for the environment and one way for better usage of the anaesthetic gases is to re-circulate them to the patient. Oxygen has to be added as well as removal of the carbon dioxide formed by the patient.
In 1777 the chemist Scheele kept bees alive in a glass jar for eight days, absorbing their CO2 with limewater. Soda lime has been used for this purpose for many years in anaesthetic applications, submarines and scuba diving.
Closed circuit or low flow anaesthesia i.e. the circle system, have become the most popular breathing system in the developed countries today.
Just above 5% CO2 is a normal level that is formed in the alveoli during respiration. This level is called the ET CO2 value (end tidal) and the inspiratory level is normally below 0.1%. These two values are normally extracted and displayed from the CO2 curve during a case.
Too high levels of CO2 in the lungs will increase the pH value of the blood (acidosis) and will, if not treated, decrease the brain activity.
In order to describe the technique relating to anaesthesia/ventilation proceedings operational window of an anaesthesia machine can be described shortly as follows.
Average system is specified for Minute Volumes from 500 milliliters to 30 liters, but normal operating conditions are between minute volumes of 3-10 liters.(Minute Volume=Tidal Volume×Respiration Rate)
The Respiratory Rate varies from 12 to 8 breaths per minute pending on the patient size.
The Tidal Volume of a patient can be anything from 300 ml to 1500 ml
Fresh gas is lead to the breathing systems about 0.5 to 3 liters/minute and the same amount is pushed out from the system. The less fresh gas is pushed in, the more CO2 needs to be extracted, CO2 that returns to the patient (FlCO2)(passes the absorber) under 0.2% or less.
The following gases are used during anaesthesia. (The CO2 value is exhaled CO2, not inspired.) The table shows percentages of different gases during anaesthesia.
MAX % (startNormal % duringCAS - NOGASof anesthesia)anesthesia124-38-9Carbon dioxide (CO2)10510024-97-2Nitric Oxide (N2O)700-707782-44-7Oxygen (O2)100307727-37-9Nitrogen(N2)780-7057041-67-5Desflurane189151-67-7Halothane5128523-86-6Sevoflurane82.526675-46-7Isoflurane51.513838-16-9Enflurane72.5alcoholtracesmethanetracesacetonetraces
As described above soda lime is widely used in the anaesthesia field to absorb CO2 from the breathing systems developed in the field.
There are different compositions of soda lime in use today but the main component in all of them are calcium hydroxide Ca(OH)2, also mentioned as slaked lime. Most of the brands also contain NaOH
Baralyme consists of 20% barium hydroxide Ba(OH)2 and 80% Ca(OH)2.CO2+H2OH2CO3  1.
CO2 in the circuit is absorbed by the water in the soda lime and forms carbonic acid.2H2CO3+2NaOH(or KOH)Na2CO3(or K2CO3)+2H2O+energy  2.
Carbonic acid reacts with the hydroxides and form carbonates (sodium or potassium carbonate), water and energy (heat).Na2CO3(or K2CO3)+Ca(OH)22NaOH(or KOH)+CaCO3  3.
These carbonates continue the reaction with the calcium hydroxide and forms calcium carbonate, also mentioned as chalk, and the alkali hydroxides.
Out from these reactions we can draw the following conclusions:    1. Water is needed to start the reaction.    2. Potassium or sodium hydroxide is used as a catalyst (not as a real catalyst since a catalyst never takes part of the reaction) since it is reformed during the reaction.    3. The energy and water formed during the second reaction can easily be detected during a case.    4. When the calcium hydroxide is consumed, the alkali bases will not be re formed and the pH will be decreased.
The decrease of pH is indicated with a dye e.g. ethyl violet (white to violet) or Mimosa Z (pink to white) to make the usage visible as a color change. This color change is however not 100% reliable since the pH can increase after some hours when the calcium hydroxides in the inner part of the soda lime granules reacts slowly and forms sodium and potassium hydroxide.
A fresh soda lime have a pH of 12 to 14 and when exhausted the pH decreases to below 10.3, which is the pH where the dye changes from white to violet. The average pH of an fully exhausted absorber is below 10. In Canada there is an upper limit of pH 12 of waste to be disposed as non hazardous material.
GE Healthcare is selling soda lime under the brand name Medisorb.
There are however several problems occurring in CO2 absorbers based on the use of soda lime in current ventilation systems. One example of the problems is a substance called Compound A. Sevoflurane can react with soda lime (no matter which commercial brand in question) and forms a nephro toxic substance called Compound A.

There are several causes that Increase the risk of Compound A forming:
1. Low fresh gas flow. This will increase the temperature and the concentration in the absorber. FDA recommends using higher fresh gas levels than 2 liters per minute to avoid Compound A.
2. Use of soda lime brands that have strong bases in them have shown to produce more Compound A than conventional soda lime.                3. High concentrations of Sevoflurane increase the risk.        4. High temperatures in the soda lime.        5. Dry soda lime        6. KOH in the Soda lime.        
Medisorb is KOH free and does not form as much Compound A as e.g. soda limes with strong bases (KOH).
Carbon monoxide, CO, is a very toxic substance that binds to the hemoglobin at the oxygen sites and and reduces the ability to transport oxygen to the body. Loss of consciousness and death may result from exposure to concentrations of 4000 ppm and higher.
Another example of the problems is CO. CO is formed in the absorbent material in higher or lower concentrations, depending on:    1. Dry soda lime increases the formation. This phenomena is also called “Monday morning effect” because of cases when the absorber is left with flushing dry gas over the weekend and that the problem was seen during startup on Monday morning.    2. High temperatures in the soda lime.
High concentrations of anaesthetic agent in order Desflurane, Enflurane, Isoflurane.
As an example of the problems of soda lime the problems occurring in connection with dry soda lime. Soda lime has to contain some water (>12%) to keep the functionality and to avoid CO and Compound A formation. It is therefore important that the ports of the compact absorbers are sealed.
Problems are caused also in that formic acid and formalin has been detected from soda lime reactions with Sevoflurane. Formaldehyde can also be seen as a impurity in sevoflurane.
There exist also two problems with the colour change in a line soda absorber. First, the colour change it is not permanent. If the absorber is stored for a while it tends to go back to its original colour and can be taken as fresh soda lime, and second, when the soda lime is exhausted, the entire absorber has not changed colour.
In the current disposable absorbers based on soda lime, the end users have problems in estimating the time of usage. Absorber's capacity is related to the way of usage. The CO2 production of patients can vary, hence the absorber absorbs different amounts of CO2/time unit. The higher the absorbance is/time unit, the less capacity the absorber has. This is due to the capability of the absorber to absorb. Normally the end user will see the rising of FiCO2 value after 5-10 hours of usage (when the absorber capacity is nearly finished) and use this as an indicator to change absorber.
In order to eliminate the problems described above research work has been carried out for some years. In this connection thoughts concerning the use of membranes for carbon dioxide removal has been suggested. One example of these thoughts is shown in PCT Patent Application WO 2004/050154 A1. In this document CO2 removal is based on using SLM technology (supported liquid membrane). This technology can so far be seen as inadequate, the selectivity is between 5 and 20 between N2O and CO2. Also the effect of anaesthetic agents (Fluranes) on the membrane has not been described in the document mentioned above.
When taken in general membranes have become an established technology for CO2 removal since the early 80's especially in the oil and gas industry. Several different types of CO2 removal membranes exist in the market and research communities. Currently the commercial membranes are mostly polymer based. The polymer can be for instance polycarbonate, polyamide, polyamide or cellulose acetate. Unfortunately these membranes are able to separate CO2 from large molecule gases, for example CH4 with a separation factor of only 15 to 60. One of the most common applications for membrane CO2 removal is purifying of natural gases.
In order to obtain better results membranes where carriers are fixed on polymer backbone, i.e. FSC membranes (Fixed Site Carrier) have been developed. One example of these solutions is described in PCT Patent Application WO 2005/089907 A1. This document describes a solution in which fixed-site-carrier (FSC) membranes are used for the purpose of separating CO2 from large molecule gases, for example CH4 (methane). The solution described can have separation factors, i.e. selectivity CO2/CH4>1000.
Conventional membranes are insufficient due to the lack of selectivity in permeation rates—for instance N2O and CO2 cannot be distinguished (they permeate with similar speed) while separating them from CH4 (or other large molecule gases), and therefore the use of said membranes has been restricted to processes in which CO2 is separated from large molecule gases.